Power amplifiers are used in a wide variety of applications, such as in communications systems for increasing the signal strength of wireless transmissions from a basestation to a wireless handset while reducing interference. In an ideal linear power amplifier, the output power equals the input power multiplied by a constant that does not vary with the input power. In existing communications systems, the power amplifier is the final active component in a basestation transmit path, and is, unfortunately, subject to nonlinearities that add noise and cause distortion. In particular, typical amplifiers become quickly and significantly non-linear at relatively low output.
Of the existing technologies, there are at least two techniques that are used in attempting to linearize the output from the power amplifier to improve the signal handling capability of the power amplifier to enable the power amplifier to operate efficiently at higher power levels or to handle multiple carriers. These include feed-forward and digital predistortion. Both feed-forward and digital predistortion systems monitor the output of the power amplifier and use this information to ensure that the amplified signal is linear. Feed-forward systems combine signals from before and after amplification to generate a distortion-canceling signal. This signal is amplified by a separate error amplifier and combined with the power amplifier output in an attempt to create a linear result. A particular disadvantage of the feed-forward system is the expense associated with requiring a second amplifier.
In a digital predistortion system, the signal is corrected prior to being upconverted to radio frequency. That is, the signal entering the power amplifier is artificially distorted according to the inverse characteristic of the amplifier. Thus, the output signal is simply an amplified replica of the original signal before predistortion.
FIG. 1 illustrates a conventional predistortion linear power amplifier 10 that uses serial intermodulation distortion (IMD) cancellation. The predistortion linear power amplifier 10 includes a predistortor 12. A signal splitter 14, power amplifiers 16 and 20 and a combiner 18 form the main power amplifier. The power amplifiers 16 and 20 are identical.
The predistortor 12, as known in the art, is used to generate, for example, a third order intermodulation distortion signal that is used to cancel the intermodulation distortion generated by the main amplifier due to the nonlinearity. The predistortor 12 and the main power amplifier are coupled in series. The predistortor 12 introduces a gain loss for the carrier signal so more amplification stages are required to compensate for the loss. For example, the predistortor 12 generates intermodulation distortion that is 180 degrees out of phase with the intermodulation distortion generated by the main amplifier. The intermodulation distortion is effectively added to the carrier signal and subsequently split into two paths by the signal splitter 14.
The power amplifiers 16 and 20 amplify the intermodulation distortion as well as the carrier signal. Due to the nonlinearity of the power amplifiers 16 and 20, the output signal from the main amplifier includes carrier signal, intermodulation distortion generated by the predistortor 12 and the intermodulation distortion generated by the power amplifiers 16 and 20. Since the intermodulation distortion generated by the predistortor 12 has the same magnitude, but is 180 degrees out of phase compared to the distortion generated by the main amplifier, the total intermodulation distortion of the output signal is significantly reduced. The combiner 18 (e.g., quadrature coupler) then combines the split carrier signal. This arrangement is typically an open loop arrangement.
A disadvantage of existing predistortion systems is their inability to adjust for an inaccurate correction estimate, which could result in an ineffectual attempt at linearization. Additionally, existing predistortion systems are able to execute only a single adaptation algorithm at one time, possibly leading to less than optimal results.